courgettes

I have tried for the past 5yrs to grow courgettes, they start really well grown from seed and I have one or two courgettes and the leaves start to go white and rot.I have grown them in pots, grow bags and last yr I grew them in a bed to give them more space but, the same thing happens.I have tried bug guns and growing marigolds amongst them bu.t nothing works anyone have any other sensible ideas. I would be grateful